The present invention relates to a process for the preparation of spun fleeces made from thermoplastic synthetic resins, hereinafter termed thermoplastics, in which fleeces the properties, particularly the tensile strength thereof, are improved.
Spun fleeces or spunbonded webs, which are built up of virtually continuous filaments of thermoplastics which are laid down in approximately random arrangement, have been known for a considerable time. Generally, they are produced by laying down the filaments, immediately after they have been spung and stretched, mainly by means of air. The degree to which remnants of parallel fiber bundles are present varies with the lay-down method used. An ideal, completely unorientated random arrangement is mostly not achieved, so that such fleeces or webs almost always have a higher tensile strength in one direction than in the direction at right angles thereto.
For a number of applications, for example, in civil engineering, it is not the strength in one direction, but the strength in all directions, which matters. This means that when the material is used, the lowest tensile strength is the determining factor, so that the thickness of the web must also be selected in accordance with the lowest tensile strength. This in turn means that it becomes more expensive to use the fleece, a fact which stands in the way of certain large industrial applications.
United Kingdom patent specification No. 1,535,988 describes a process in which the properties of staple fiber webs wherein the individual fibers are orientated at right angles to the machine direction are improved by stretching the webs first in the longitudinal direction, then needle-punching them, then again stretching them in the longitudinal direction and finally stretching them in the transverse direction. This increases the dimensional stability and strength of these webs.
Also, it is known from United Kingdom patent specification No. 1,371,863 that in the case of non-consolidated randomly arranged staple fiber webs consisting of relatively short fibers, which have been provided with a regular pattern by means of mechanical or fluid forces, the transverse tensile strength may be improved by stretching in the transverse direction, accompanied by shrinkage in the longitudinal direction, without this stretching destroying the pattern consisting of regular thick and thin areas. Rather, the pattern may be completely reconstituted by another aftertreatment with fluid forces, which bring about a reorientation of the relatively short fibers.
Finally, West German Offenlegungsschrift No. 1,635,634 discloses a method of improving the longitudinal tensile strength of webs, which as a result of plaiting-down have a strong orientation in the transverse direction, by stretching them in the longitudinal direction during needle-punching, This stretching, which at the same time results in an uncontrollable reduction in width, is intended to have the effect that the fiber pile, which in the plaiter is laid down with the layers at angle of 10.degree. to 15.degree. to one another, is so distorted during the first needle-punching operation that the fibers ultimately settle at an angle of 45.degree. to one another, and are fixed in this position.
This process, which may be carried out, during needle-punching, only by breaking it down into numerous individual small stretching steps, requires expensive equipment, since, for example, the needle-punching machine must work with a low punching speed but a high output speed and furthermore should execute a side-to-side motion since otherwise stripes are produced in the web. This Offenlegungsschrift also points out that simple stretching of the plaited-down web is not possible since it results in the formation of thin areas which tear on further stretching.
Stretching to improve the properties also has been proposed for webs of continuous filaments. According to United Kingdom patent specification No. 1,213,441, webs which are welded or glue-bonded at the cross-over points are stretched, in at least one direction, to the extent that the surface area increases by a factor of up to about 15. Since the cross-over points in the webs used for this process are rigidly fixed, this treatment results in a stretching of the individual filaments, which vary greatly in individual gauge, and results in fluctuations in gauge, of the individual filaments, by a power of ten. Stretching in this process is effected over a heated brake pad.
It now has been found that the tensile strengths of a spun fleece in which the filaments are in approximately random arrangement may be substantially equalized in directions at right angles to one another, the lower tensile strength in one of the two directions being substantially increased, without however the filaments themselves being stretched and the filament gauge becoming non-uniform, if a needle-punched fleece is stretched, at an elevated temperature, in the direction which has the lower tensile strength.
As used herein the term "random arrangement" is intended to mean the substantial absence of any anisotropy in the arrangement of the individual filaments
As a result of this stretching step, the lower tensile strength is increased though at the same time the surface area of the fleece is enlarged at the expense of the weight per square meter. The fact that nevertheless a higher minimum tensile strength is achieved now offers the possibility of substantially more economical use of the fleece, above all in underground civil engineering, such as road making, tunnel construction, construction of embankments and hydraulic civil engineering, since what matters in these applications is virtually only the force-elongation characteristics, but not the weight of the web per square meter, and accordingly larger surfaces may be covered with the same weight of a fleece material.
The fact that a needle-punched fleece, in which the cross-over points are not so consolidated that they will not open up, may be reinforced is surprising since it was to be expected that any thin areas which might be present would be accentuated or that holes might even appear. However, this is not the case. On the contrary, a more uniform distribution of the randomly arranged filaments is achieved, with filaments lying in loops passing into the stretched position with increasing degree of stretching, and thereby imparting a higher strength to the fleece. However, this may be achieved only if the stretching is carried out within a certain temperature range which depends on the crystallite melting point.